Rhodium(III)-Catalyzed C-H/N-H Alkyne Annulation of Nonsymmetric 2-Aryl (Benz)imidazole Derivatives: Photophysical and Mechanistic Insights

Ruthenium(II)-Catalyzed Sequential C-H/N-H Alkene Annulation Cascade of Phenanthroimidazoles: Synthesis and Photophysical Studies

We report ruthenium(II)-catalyzed sequential C-H/N-H alkenylation cascade of phenanthroimidazole and alkenes to form novel phenanthroimidazoisoindol acrylates via dual C-H activation and aza-Michael reaction. The two nitrogen atoms of the imidazole ring act as directing groups for regioselective dual sequential ortho C-H activation. These polycyclic N-heterocycles were evaluated for their photophysical properties.

Ru(II)-Catalyzed [4 + 2]-Annulation of 2-Alkenyl/Arylimidazoles with N-Substituted Maleimides and 1,4-Naphthoquinones: Access to Imidazo-Fused Polyheterocycles

Synthesis of imidazo-fused polyheterocyclic molecular frameworks, viz. imidazo[1,2-a]pyrrolo[3,4-e]pyridines, imidazo[2,1-a]pyrrolo[3,4-c]isoquinolines, and benzo[g]imidazo[1,2-a]quinoline-6,11-diones, has been achieved by the ruthenium(II)-catalyzed [4 + 2] C-H/N-H annulation of 2-alkenyl/2-arylimidazoles with N-substituted maleimides and 1,4-naphthoquinones. The developed protocol is operationally simple, exhibits broad substrate scope with excellent functional group tolerance, and provides the desired products in moderate to good yields. The mechanistic studies suggest that the reaction involves the formation of a C-C bond through Ru-catalyzed C(sp2)-H bond activation followed by intramolecular cyclization.

Ruthenium-Catalyzed Oxidative Synthesis of N-(2-triazine)indoles by C-H Activation

1,3,5 triazines, especially indole functionalized triazine derivatives, exhibit excellent activities, such as anti-tumor, antibacterial, and anti-inflammatory activities. Traditional methods for the synthesis of N-(2-triazine) indoles suffer from unstable materials and tedious operations. Transition-metal-catalyzed C-C/C-N coupling provides a powerful protocol for the synthesis of indoles by the C-H activation strategy. Here, we report the efficient ruthenium-catalyzed oxidative synthesis of N-(2-triazine) indoles by C-H activation from alkynes and various substituted triazine derivatives in a moderate to good yield, and all of the N-(2-triazine) indoles were characterized by ¹H NMR, ¹³C NMR, and HRMS. This protocol can apply to the gram-scale synthesis of the N-(2-triazine) indole in a moderate yield. Moreover, the reaction is proposed to be performed via a six-membered ruthenacycle (II) intermediate, which suggests that the triazine ring could offer chelation assistance for the formation of N-(2-triazine) indoles.

First report of trans-A2B-corrole derived from a lapachone derivative: photophysical, TD-DFT and photobiological assays

In this work, the synthesis, characterization and photophysical assays of a new trans-A₂B-corrole derivative from the naturally occurring quinone are described. β-Lapachone is a naturally occurring quinoidal compound that provides highly fluorescent heterocyclic compounds such as lapimidazoles. The new trans-A₂B-corrole compound was obtained from the reaction between 2,3,4,5,6-(pentafluorophenyl)dipyrromethane and the lapimidazole bearing an aldehyde group. The dyad was characterized by high-resolution mass spectrometry (HRMS), NMR spectroscopy (¹H, COSY 2D, HMBC, ¹⁹F), FT-IR, UV-vis, steady-state and time-resolved fluorescence, electrochemical studies (CV), TD-DFT analysis and photobiological experiments, in which includes aggregation, stability in solution, photostability and partition coefficients assays. Finally, ROS generation assays were performed using 1,3-diphenylisobenzofuran (DPBF) method and the presented compound showed significant photostability and singlet oxygen production.

Click Triazole as a Linker for Pretargeting Strategies: Synthesis, Docking Investigations, Fluorescence Diagnosis, and Antibacterial Action Studies

In this study, three compounds A1, A2, and A3 and fluorescent probes T1, T2, T3, and T4 were designed and synthesized. ¹H NMR, ¹³C NMR, and MS characterization and elemental analysis were used to confirm A1-A3 and T1-T4. A1-A3 and T1-T4 formed diagnostic molecules by "click" reactions. A1-A3 and T1-T4 did not significantly increase cell death at concentrations of 80 μmol/L. Preliminary screening of the compounds for antibacterial activity revealed that A2 has better antibacterial activity against Agrobacterium tumefaciens. The synthesized compounds and fluorescent probes can be targeted and combined in the physiological condition to form diagnostic molecules for fluorescence detection of Agrobacterium tumefaciens. The binding sites of A1-A3 were deduced theoretically using the AutoDock Vina software docking tool. Further study of the mechanism of the antibacterial action of these compounds is likely to identify new agents against resistant bacterial strains.

Preorganized Polyaromatic Soft Terdentate Hosts for the Capture of [Ln(β-diketonate)3 ] Guests in Solution

The concept of preorganization is famous in coordination chemistry for having transformed flexible bidentate 2,2'-bipyridine scaffolds into rigid 1,10-phenanthroline platforms. The resulting boosted affinities for d-block cations has successfully paved the way for the design of a wealth of functional complexes, devices and materials for analysis and optics. Its extension toward terdentate homologues adapted for the selective complexation of f-block cations with larger coordination numbers remains more overlooked. The resulting rigidification of 2,6-bis(1-methyl-1H-benzo[d]imidazol-2-yl)pyridine ligands (L1-L7) produces the highly preorganized and extended polyaromatic benzo[4',5']imidazo[1',2' : 1,2]pyrido[3,4-b]benzo[4,5]imidazo[1,2-h][1,7]naphthyridines (L8-L11) receptors, which offer some novel and rare opportunities for efficiently complexing trivalent lanthanides with polyaromatic soft terimine ligands. The crystal structures of the stable heteroleptic [LkLn(hfac)₃ ] adducts (Lk=L1, L8, L9; Ln=La, Eu, Gd, Er, Yb, Y; H-hfac=1,1,1,5,5,5-hexafluoropentane-2,4-dione) show a drastic decrease in the Ln-N bond valences upon replacement of the flexible ligand L1 with its preorganized counterparts L8 and L9. This points to a limited match between the preorganized cavity and the entering [Ln(hfac)₃ ] lanthanide containers. However, thermodynamic studies conducted in dichloromethane reach the opposite conclusion, with an improved affinity, by up to three orders of magnitude for catching Ln(hfac)₃ when L1 is replaced by the preorganized L8-L9 receptors. The key to the enigma lies in the removal of the energy penalty which accompanies the formation of flexible [L1Ln(hfac)₃ ] complexes in solution. This driving force overcomes the poor match between the preorganized terdentate N^∩ N^∩ N cavity in L8 and L9 and the size of trivalent lanthanides. As planned, the rigid, planar and extended π-conjugated system found in L8 and L9 shifts the ligand-centered absorption bands by about 5000 cm^-1 toward lower energies, a crucial point if these stable [L8Ln(hfac)₃ ] and [L9Ln(hfac)₃ ] platforms have to be considered for the visible sensitization of luminescent lanthanides in metallopolymers.

Pd(II)-Catalyzed Synthesis of Furo[2,3-b]pyridines from β-Ketodinitriles and Alkynes via Cyclization and N-H/C Annulation

A Pd(II)-catalyzed synthesis of furopyridines has been developed from β-ketodinitriles and alkynes via an unusual N-H/C annulation. The participation of both the nitrile groups and the concurrent construction of furan and pyridine rings through the formation of C-C, C═C, C-O, C-N, and C═N bonds are the important features. The synthetic applicability is further demonstrated through a series of postsynthetic alterations.

Rh-Catalyzed [4 + 2] Annulation with a Removable Monodentate Structure toward Iminopyranes and Pyranones by C-H Annulation

The Rh-catalyzed reactions of N-pyridinyl enaminones with internal alkynes leading to the synthesis of iminopyranes via a key C-H bond activation and subsequent tautomeric O-H bond cleavage are reported. Moreover, the pyridine ring in the amino group acts as an auxiliary monodentate site for this annulation and can be easily removed by a simple hydrolysis to afford pyranones.

Regioselective alkylation of 2,4-dihydroxybenzyaldehydes and 2,4-dihydroxyacetophenones

We report a cesium bicarbonate-mediated alkylation of 2,4-dihydroxybenzyaldehyde and 2,4-dihydroxyacetophenone to generate 4-alkylated products in acetonitrile at 80 °C with excellent regioselectivity, up to 95% isolated yields, and broad substrate scope.

Recent Advances in the Synthesis of Isoquinoline-Fused Benzimidazoles

This review includes recent developments in the synthesis of benzo[4,5]imidazo[2,1-a]isoquinolines with particular attention given to categorizing protocols based on the structural features of the ring architecture and crystallographically characterized reaction products.

Multiple Reaction Pathways of Eight-Membered Rhodacycles in Rh-Catalyzed Annulations of 2-Alkenyl Phenols/Anilides with Alkynes

Density functional theory calculations were performed to study the competing pathways of rhodacycle intermediates generated in Rh(III)-catalyzed annulations of 2-alkenyl phenols and 2-alkenyl anilides with alkynes. The results show that the multiple pathways of eight-membered rhodacycles can be subtly tuned to give specific cyclic products. The seven-membered oxacyclic and spirocyclic products from 2-alkenyl phenols are formed by favoring the pathway of dissociating the Rh-O bond of O-contained rhodacycles, which are followed by antarafacial nucleophilic attack. The indoline product from 2-alkenyl anilides is generated through the pathway of intramolecular olefin migratory insertion of the N-contained rhodacycle.

Electricity-induced micro-flow C–H/N–H alkyne annulation: a greener approach to access heteroaromatic compounds

Fast access to structurally diverse heteroaromatic compounds is demonstrated in an integrated continuous-flow manner without employing any expensive oxidant.